Mechanistic Changeover for the Water Substitution on fac-[(CO)₃Re(H₂O)₃]⁺ Revealed by High-Pressure NMR

Abstract

The complex formation in water between the stable tricarbonyltriaqua fac-[(CO)(3)Re(H(2)O)(3)](+) (1) complex and N- and S-donor ligands has been studied by high-pressure (1)H NMR. Rate and equilibrium constants for the formation of [(CO)(3)Re(Pyz)(H(2)O)(2)](+), [(CO)(3)(H(2)O)(2)Re(mu-Pyz)Re(H(2)O)(2)(CO)(3)](2+), [(CO)(3)Re(THT)(H(2)O)(2)](+), and [(CO)(3)Re(DMS)(n)()(H(2)O)(3-n)](+) (n = 1-3) (Pyz = pyrazine, THT = tetrahydrothiophene, DMS = dimethyl sulfide) have been determined and are in accord with previous results (Salignac, B.; Grundler, P. V.; Cayemittes, S.; Frey, U.; Scopelliti, R.; Merbach, A. E.; Hedinger, R.; Hegetschweiler, K.; Alberto, R.; Prinz, U.; Raabe, G.; Kölle, U.; Hall, S. Inorg. Chem. 2003, 42, 3516). The calculated interchange rate constant k(1)' (Eigen-Wilkins mechanism) increases from the hard O- and N-donors to the soft S-donors, as exemplified by the following series: TFA (trifluoroacetate) (k(1)' = 2.9 x 10(-3) s(-1)) < Br(-) < CH(3)CN < Pyz < THT < DMS < TU (thiourea) (k(1)' = 41.5 x 10(-3) s(-1)). On the other hand, values remain close to that of water exchange k(ex) on 1 (k(ex) = 6.3 x 10(-3) s(-1)). Thus, an I(d) mechanism was assigned, suggesting however the possibility of a slight deviation toward an associatively activated mechanism with the S-donor ligands. Activation volumes determined by high-pressure NMR, for Pyz as Delta V(++)(f,1) = +5.4 +/- 1.5, Delta V(++)(r,1) = +7.9 +/- 1.2 cm(3) mol(-)(1), for THT as Delta V(++)(f,1) = -6.6 +/- 1, Delta V(++)(r,1) = -6.2 +/- 1 cm(3) mol(-1), and for DMS as Delta V(++)(f,1) = -12 +/- 1, Delta V(++)(r,1) = -10 +/- 2 cm(3) mol(-1) revealed the ambivalent character of 1 toward water substitution. Hence, these findings are interpreted as a gradual changeover of the reaction mechanism from a dissociatively activated one (I(d)), with the hard O- and N-donor ligands, to an associatively activated one (I(a)), with the soft S-donor ligands.

Ligand exchange and complex formation kinetics studied by NMR exemplified on fac-[(CO)3M(H2O)]+ (M=Mn, Tc, Re)

“…An interesting feature of the complex formation of fac-[(CO) 3 Re(H 2 O) 3 ] + with the ligands studied is the strong increase in the rate constant for the back reaction, k r,1 [62]. From the pyrazine (Pyz) ligand to Br − it increases by nearly three orders of magnitude (Table 5).…”

“…As an example, for the complexation of the Re I compound with DMS the following constants have been determined (T = 298 K): 8. [62]. In this example, where 1 H resonances are not overlapping and the kinetics is slow, all rate constants for formation and dissociation can be determined.…”

“…Assuming second-order kinetics, the variation of the concentrations of all observed species can be calculated as functions [62]. In this simultaneous fit the ratios of k f,i and k r,i have been fixed to the equilibrium constants K i determined after full equilibration.…”

“…The most rate and equilibrium constants on complex formation on tricabonyl metal triaqua complexes are available on fac-[(CO) 3 Re(H 2 O) 3 ] + [46,62]. A series of N-and S-bonded ligands, neutral and negatively charged ligands have been studied (Table 5).…”

“…From the pyrazine (Pyz) ligand to Br − it increases by nearly three orders of magnitude (Table 5). This marked leaving group effect represents the changing nucleofugal ability of each species, which is inversely related to their basic character [62]. Bromide with a pK a (HBr) = −4.7 is the fastest leaving group and Pyz with pK a (Pyz) = 0.6 is the slowest one.…”